inner join on range criteria: unable to use index?

[Joseph Fisk]

Hello,

Even after searching for a while I'm having trouble understanding why this query is not faster:

select * from s inner join r on r.tstart >= s.chrstart and r.tend <= s.chrstop and r.tname=s.chr and r.strand=s.chrstrand;

tstart/tend/chrstart/chrstop are all bigint, and of the columns involved in the join, they are the most selective.

I would expect mysql to scan the smaller table, and then take advantage of a multi column index on the larger table on (start, stop, chr, strand), which is in order from most to least selective.

But mysql won't use that index. Why not?